1. Field of the Invention
This invention relates generally to a device for controlling the tension in a continuous web of paper, foil, cloth or the like material fed through a web processing machine, and more particularly to such a device which makes use of a web-tension sensing roller carrying fluid pressure piston and cylinder units for dampening pivotal movement of such roller.
2. Description of the Prior Art
Web processing machines, including printing machines, cutting machines, etc., often include devices for controlling the tension of the web fed therethrough. The continuous web to be processed is led over an appropriate arrangement of guide rollers forming a web loop at which the web passes over a web-tension sensing roller rotatably mounted on rod members for pivotal movement toward and away from the idler rollers. During operation of the machine, a change of the tension or stretch of the continuous web effects a change in the size of the web loop thereby altering the position of the sensing roller. A brake device on the web feed roll is connected to such rod members so as to be controlled by the sensing roller for controlling the feed rate of the web to maintain a predetermined tension thereof.
Normally, however, the sensing roller or those rod members to which the roller is rotatably mounted within the machine are loaded by weights in such a manner that the position of the weights can be partially altered. The continuous web to be processed is accordingly tensioned to an amount depending on the weight of the sensing roller and the weights attached thereto. However, the larger the weights acting on the web, the larger are the masses which must be moved with a changing of the web loop. And, in order to render the control of the weights as sensitive as possible to the web, the masses of the weights must be maintained as low as possible without changing the force of the sensing roller resting on the continuous web.
Among prior art, U.S. Pat. No. 3,083,887 to Huck discloses a web tension control mechanism wherein force is applied to the sensing roller by means of a pressure cylinder in such a manner that the force on the sensing roller is effected by the diameter of the pressure cylinder or its piston, and from the pressure to which the cylinder and piston unit is subjected. The disadvantage of such a prior art mechanism is that the dampening effect on the sensing roller provided by such a piston and cylinder unit is principally dependent on the dimensions of such unit which, of course, remains constant during the operation of the machine. Another problem which such prior art mechanism fails to solve is with respect to the loading of the web processing machine, i.e., at the time a new continuous web is introduced into the machine and starts to stretch when the machine is started up. This involves a special problem which must be reckoned with since, when a new continuous web is introduced into the machine, it is generally not subject to any tension at that time so that the web loop in rolling contact with the sensing roller is of a maximum size. Therefore, in accordance with the standard arrangement, the brake which is associated with the web feed roller and which is controlled by the sensing roller would be applied as much as possible since the maximum size of the loop must be taken up and the tension in the continuous web must be increased by additionally braking the feed roll. Also, at the start of operation of the machine, the continuous web begins stretching and clinging to the sensing roller since, because of its mass, the sensing roller functions as though rigidly mounted and since, at the same time, the web feed roll is prevented from unwinding because of the condition of the sensing roller. And, if the machine is quickly started up, a high tension is developed almost immediately in the continuous web which can reach such a degree of tension as to cause web rupture. In order to moderate this effect, the sensing roller has been connected with a piston and cylinder unit or a shock absorber as, for example, disclosed by U.S. Pat. No. 3,083,887. However, as mentioned hereinabove, such a shock absorber has a dampening effect which is only dependent on the dimension thereof which dimension remains constant throughout the operation of the machine.